Kinetic Resolution of β-Branched Aldehydes through Peptide-Catalyzed Conjugate Addition Reactions

The catalytic kinetic resolution of racemic β-branched aldehydes offers a straightforward stereoselective entry to aldehydes and addition products. Yet, control over stereoselectivity is difficult due to the conformational flexibility of β-branched aldehydes. Here, we show that the peptide catalyst H-dPro-αMePro-Glu-NH2 resolves β-branched aldehydes through reaction with nitroolefins and provides γ-nitroaldehydes with three consecutive stereogenic centers in high yields and stereoselectivities. Kinetic, NMR spectroscopic, and computational studies provided insights into the selectivity-determining step and origins of the kinetic resolution.

Chromium-Catalyzed Alkene Isomerization with Switchable Selectivity

We report a single additive-responsive chromium-catalyzed system for selectively producing either of two different internal alkene isomers. The chromium catalyst, in the presence of HBpin/LiOtBu, enables the isomerization of alkenes over multiple carbon atoms to give the most thermodynamically stable isomers. The same catalyst allows for the selective isomerization of terminal alkenes over one carbon atom without an additive, exhibiting efficient and controllable alkene transposition selectivity.

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar-Ir-H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk-Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir-C bond followed by C-H reductive elimination to afford the hydrofunctionalization products site-selectively.

Enantioconvergent transformations of secondary alcohols through borrowing hydrogen catalysis

Direct substitution of readily available alcohols is recognized as a key research area in green chemical synthesis. Starting from simple racemic secondary alcohols, the achievement of catalytic enantioconvergent transformations of the substrates will be highly desirable for efficient access to valuable enantiopure compounds. To accomplish such attractive yet challenging transformations, the strategy of the enantioconvergent borrowing hydrogen methodology has proven to be uniquely effective and versatile. This review aims to provide an overview of the impressive progress made on this topic of research that has only thrived in the past decade. In particular, the conversion of racemic secondary alcohols to enantioenriched chiral amines, N-heterocycles, higher-order alcohols and ketones will be discussed in detail.

Preparation of Distant Quaternary Carbon Stereocenters by Double Selective Ring-Opening of 1,1-Biscyclopropyl Methanol Derivatives

The diastereoselective double carbometalation reaction of cyclopropenes provides, in a single-pot operation, two ω-ene-[1,1]-bicyclopropyl ester derivatives. One regioisomer then undergoes a Pd-catalyzed addition of aryl iodide to provide skipped dienes possessing several distant stereocenters including two congested quaternary carbon centers with excellent diastereoselectivity.

Remote Site-Selective Asymmetric Protoboration of Unactivated Alkenes Enabled by Bimetallic Relay Catalysis

A remote C(sp³ )-H bond asymmetric borylation of unactivated alkenes was achieved by bimetallic relay catalysis. The reaction proceeded through reversible and consecutive β-H elimination/olefin insertion promoted by CoH species generated in situ, followed by copper-catalyzed asymmetric protoboration. The use of this synergistic Co/Cu catalysis protocol allowed the enantioselective protoboration of various unactivated terminal alkenes and internal alkenes, as well as an unrefined mixture of olefin isomers, at the distal less-reactive β-position to a functional group, leading to chiral organoboronates.

Synthesis of α,γ-Chiral Trifluoromethylated Amines through the Stereospecific Isomerization of α-Chiral Allylic Amines

Chiral γ-branched aliphatic amines are present in a large number of pharmaceuticals and natural products. However, enantioselective methods to access these compounds are scarce and mainly rely on the use of designed chiral transition-metal complexes. Herein, we combined an organocatalytic method for the stereospecific isomerization of chiral allylic amines with a diastereoselective reduction of the chiral imine/enamine intermediates, leading to γ-trifluoromethylated aliphatic amines with two noncontiguous stereogenic centers, in excellent yields and high diastereo- and enantioselectivities. This approach has been used with primary amine substrates. This approach also provides a new synthetic pathway to chiral trifluoromethylated scaffolds, of importance in medicinal chemistry. Additionally, a gram-scale reaction demonstrates the applicability of this synthetic procedure.

Electrocatalytic Isomerization of Allylic Alcohols: Straightforward Preparation of β-Aryl-Ketones

Electrochemical synthesis has been rapidly developing over the past few years. Here, we report a practical and eco-friendly electrocatalytic isomerization of allylic alcohols to their corresponding carbonyl compounds. This reaction can be carried out in undivided cells without the addition of external chemical oxidants and metal catalysts. Moreover, this reaction features a broad substrate scope including challenging allylic alcohols bearing tri- and tetra-substituted olefins and affords straightforward access to diverse β-aryl-ketones. Mechanistic investigations suggest that the reactions proceed through a radical process. This study represents a unique example in which electrochemistry enables hydrogen atom transfer in organic allylic alcohol substrates using a simple organocatalyst.

Desymmetrizing Isomerization of Alkene via Thiazolinyl Iminoquinoline Cobalt Catalysis

We report a cobalt-catalyzed desymmetrizing isomerization of exo-cyclic alkenes to generate chiral 1-methylcyclohexene derivatives with good yields and enantioselectivities. A novel chiral thiazolinyl iminoquinoline ligand and its cobalt complex were designed and synthesized to control the establishment of tertiary or quaternary carbon centers at a remote position. This protocol is operationally simple, and a model for the stereochemical outcome has been proposed.

Asymmetric Hydrogenation of Racemic Allylic Alcohols via an Isomerization-Dynamic Kinetic Resolution Cascade

Prochiral racemic allylic alcohols are converted to enantioenriched chiral alcohols bearing adjacent stereocenters catalyzed by a diamine diphosphine Ru complex in the presence of tBuOK. The protocol features a broad substrate scope (56 examples) and high diastereo- and enantioselectivities (up to >99:1 dr, >99% ee) and could be applied to the synthesis of enantioenriched chromane and indane compounds. Mechanistic studies suggest that the reaction proceeds via tBuOK-promoted allylic alcohol isomerization followed by Ru-catalyzed hydrogenative dynamic kinetic resolution.

Palladium-Catalyzed α-Amino C-H Functionalization via Isomerization of α,β-Unsaturated Carbonyls

Palladium-catalyzed regioselective α-amino C-H functionalization via the isomerization of α,β-unsaturated carbonyls including esters, ketones, and amides has been established, providing an easy access to a wide array of tricyclic 1,2,3,4-tetrahydro-b-carbolines, azepinoindoles, 2,3,4,5-tetrahydro-1H-pyrido[4,3-b]indoles, 4,5,6,7-tetrahydro-1H-pyrrolo[2,3-c]pyridines, 1,2,3,4-tetrahydropyrazino-[1,2-a]indoles, pyran-fused indoles, and tetrahydroisoquinolines in good to excellent yields. This transformation showed high regioselectivity, excellent functional group tolerance, and scalability. Moreover, this methodology was also employed as the key step for the total synthesis of desbromoarborescidines A, B, and C. Preliminary mechanistic studies revealed that the palladium catalyst not only formed [Pd-H] to promote the isomerization of α,β-unsaturated carbonyls but also played a role as a Lewis acid for the final protonation/cyclization.

Redox-neutral remote amidation of alkenyl alcohols via long-range isomerization/transformation

A facile and straightforward approach for the construction of amides via redox-neutral Ru-catalyzed cross-coupling reaction of long-range alkenyl alcohols with amines to realize remote site-selective functionalization has been developed.

Cobalt-Catalyzed Desymmetric Isomerization of Exocyclic Olefins

Chiral cyclic olefins, 1-methylcyclohexenes, are versatile building blocks for the synthesis of pharmaceuticals and natural products. Despite the prevalence of these structural motifs, the development of efficient synthetic methods remains an unmet challenge. Herein we report a novel desymmetric isomerization of exocyclic olefins using a series of newly designed chiral cobalt catalysts, which enables a straightforward construction of chiral 1-methylcyclohexenes with diversified functionalities. The synthetic utility of this methodology is highlighted by a concise and enantioselective synthesis of a natural product, β-bisabolene. The versatility of the reaction products is further demonstrated by multifarious derivatizations.

Stereoselective tandem iridium-catalyzed alkene isomerization-cope rearrangement of ω-diene epoxides: efficient access to acyclic 1,6-dicarbonyl compounds

The Cope rearrangement of 2,3-divinyloxiranes, a rare example of epoxide C-C bond cleavage, results in 4,5-dihydrooxepines which are amenable to hydrolysis, furnishing 1,6-dicarbonyl compounds containing two contiguous stereocenters at the 3- and 4-positions. We employ an Ir-based alkene isomerization catalyst to form the reactive 2,3-divinyloxirane in situ with complete regio- and stereocontrol, which translates into excellent control over the stereochemistry of the resulting oxepines and ultimately to an attractive strategy towards 1,6-dicarbonyl compounds.

Using Bases as Initiators to Isomerize Allylic Alcohols: Insights from Density Functional Theory Studies

Allylic alcohols, as common and readily available building blocks, could be converted into many widely used carbonyl compounds through isomerization reactions. However, these processes often involve expensive transition metal (TM) complexes as the catalyst. What is the bottleneck in the mechanism when no TM is used? In this study, density functional theory (DFT) was employed to explore the mechanistic patterns of allylic alcohols catalyzed using bases, such as KOH, NaOH, LiOH, tBuOK, tBuONa, tBuOLi, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine, and 1,8-diazabicyclo[5.4.0]undec-7-ene. Our results show that bases containing metal cations follow the metal cation-assisted (MCA) mechanism, whereas organic bases without metal cations follow the ion pair-assisted (IPA) mechanism. The catalytic efficiency of bases containing metal cations is higher than that of bases without metal cations, indicating that metal cations play an important role in the reaction. Additionally, the modulation of substituents R₁ and R₂ in the substrate reveals that electron-withdrawing groups are favorable for C-H bond cleavage, and electron-donating groups are favorable for hydrogen transfer. To better understand these patterns, we applied the DFT and information-theoretic approach (ITA) to examine the impact of bases and substrate substituents on the reactivity of allylic alcohol isomerization. This work should provide a much-needed theoretical guidance to design better non-TM catalysts for the isomerization of allylic alcohols and their derivatives.

Assessing Methodologies to Synthesize α-Sulfenylated Carbonyl Compounds by Green Chemistry Metrics

α-Sulfenylated carbonyl compounds are important both as active pharmaceutical ingredients and as intermediates in organic synthesis. Owing to their relevance in synthetic organic chemistry, this Minireview focuses on assessing the most relevant synthetic procedures based on green chemistry metrics. The Minireview starts with the traditional routes and then focuses on more recently developed methodologies. These routes include sulfenylating reagents using organocatalysis, cross-dehydrogenative couplings using in situ halogenations to prevent reactive intermediates in high concentrations, oxidative couplings using terminal oxidants such as DDQ or TEMPO, and redox-neutral couplings using transition metal catalysis. These methodologies have been evaluated on the basis of atom economy, E factor, and the safety and toxicity of the transformations and the solvents used. Besides using green metrics to evaluate these novel methodologies, the synthetic utility is also assessed with regard to the availability of starting materials and the generality of the reactions. This Minireview aims to inspire researchers to apply green assessments to other methodologies and also for them to take measures to increase the greenness of their developed transformations.

Stereoselective Access to Fully Substituted Aldehyde-Derived Silyl Enol Ethers by Iridium-Catalyzed Alkene Isomerization

An in situ generated cationic Ir-catalyst isomerizes simple allylic silyl ethers into valuable, fully substituted aldehyde-derived silyl enol ethers. Importantly, by judicious choice of substrate, either of the two possible stereoisomers of a given enolate derivative is accessible with complete stereoselectivity. One-pot isomerization-aldol and isomerization-allylation processes illustrate the synthetic utility of this method.

Photoredox-Catalyzed Isomerization of Highly Substituted Allylic Alcohols by C-H Bond Activation

Photoredox-catalyzed isomerization of γ-carbonyl-substituted allylic alcohols to their corresponding carbonyl compounds was achieved for the first time by C-H bond activation. This catalytic redox-neutral process resulted in the synthesis of 1,4-dicarbonyl compounds. Notably, allylic alcohols bearing tetrasubstituted olefins can also be transformed into their corresponding carbonyl compounds. Density functional theory calculations show that the carbonyl group at the γ-position of allylic alcohols are beneficial to the formation of their corresponding allylic alcohol radicals with high vertical electron affinity, which contributes to the completion of the photoredox catalytic cycle.

A Catalytic Dual Isomerization/Allylboration Sequence for the Stereoselective Construction of Congested Secondary Homoallylic Alcohols

A catalytic sequence for the diastereo- and enantioselective preparation of homoallylic alcohols with an adjacent quaternary (stereo)center is reported. The one-pot process relies on the use of a single (achiral or chiral) iridium complex to catalyze the concomitant isomerization of primary allylic alcohols and homoallylboronates into (chiral) aldehydes and allylboronates, respectively. In the same flask, a chiral Brønsted acid is added next to engage the isomerization products into a stereocontrolled allylboration reaction. Structural variations have been performed on both the allylic alcohols and the homoallylboronates. This mild process affords an array of stereochemically congested and complex chiral secondary homoallylic alcohols in high yield, excellent diastereoselectivity, and usually high enantioselectivity.

Catalytic Approach toward Chiral P,N-Chelate Complexes Utilizing the Asymmetric Hydrophosphination Protocol

A synthetic procedure for obtaining a chiral P,N ligand was developed by exploiting the versatility of the asymmetric hydrophosphination protocol catalyzed by a phosphapalladacycle complex. The addition of the synthesized ligand to various metal sources led to the generation of chiral and enantioenriched chelate complexes, which can be useful prototypes for catalyst design in the future. The resulting coordination compounds were comprehensively characterized by solid-state (X-ray crystallography) and solution-based (one- and two-dimensional NMR spectroscopy) techniques and natural bond orbital (density functional theory) analysis to determine their structural and key electronic features.

One-Pot Conversion of Allylic Alcohols to α-Methyl Ketones via Iron-Catalyzed Isomerization-Methylation

A one-pot iron-catalyzed conversion of allylic alcohols to α-methyl ketones has been developed. This isomerization-methylation strategy utilized a (cyclopentadienone)iron(0) carbonyl complex as precatalyst and methanol as the C1 source. A diverse range of allylic alcohols undergoes isomerization-methylation to form α-methyl ketones in good isolated yields (up to 84% isolated yield).

High-Atom Economic Approach To Prepare Chiral α-Sulfenylated Ketones

Chiral α-sulfenylated ketones are versatile building blocks, although there are still several limitations with their preparation. Here we report a new two-step procedure, consisting of Pd-catalyzed hydrothiolation of propargylic alcohols followed by an enantioselective Rh isomerization of allylic alcohols. The isomerization reaction is the key step for obtaining the ketones in their enantioenriched form. The new methodology has a high atom economy and induces good to high levels of enantioselectivity; no waste is produced. A mechanism involving a Rh-hydride-enone intermediate is proposed for the isomerization reaction.

Enantioconvergent Biocatalytic Redox Isomerization

Alcohol dehydrogenases can act as powerful catalysts in the preparation of optically pure γ-hydroxy-δ-lactones by means of an enantioconvergent dynamic redox isomerization of readily available Achmatowicz-type pyranones. Imitating the traditionally metal-mediated "borrowing hydrogen" approach to shuffle hydrides across molecular architectures and interconvert functional groups, this chemoinspired and purely biocatalytic interpretation effectively expands the enzymatic toolbox and provides new opportunities in the assembly of multienzyme cascades and tailor-made cellular factories.

Pd-Catalyzed Selective Remote Ring Opening of Polysubstituted Cyclopropanols

The distant functionalization of ω-ene cyclopropanols is induced by a Pd-catalyzed Heck reaction triggering a "metal-walk" and selective ring-opening of the three-membered ring. This approach provides a new class of acyclic aldehydes possessing concomitantly a stereodefined double bond and a quaternary carbon stereocenter α to the carbonyl group.

Encapsulation of Crabtree's Catalyst in Sulfonated MIL-101(Cr): Enhancement of Stability and Selectivity between Competing Reaction Pathways by the MOF Chemical Microenvironment

Crabtree's catalyst was encapsulated inside the pores of the sulfonated MIL-101(Cr) metal-organic framework (MOF) by cation exchange. This hybrid catalyst is active for the heterogeneous hydrogenation of non-functionalized alkenes either in solution or in the gas phase. Moreover, encapsulation inside a well-defined hydrophilic microenvironment enhances catalyst stability and selectivity to hydrogenation over isomerization for substrates bearing ligating functionalities. Accordingly, the encapsulated catalyst significantly outperforms its homogeneous counterpart in the hydrogenation of olefinic alcohols in terms of overall conversion and selectivity, with the chemical microenvironment of the MOF host favouring one out of two competing reaction pathways.

Asymmetric synthesis of γ-branched amines via rhodium-catalyzed reductive amination

Amines bearing γ-stereocenters are highly important structural motifs in many biologically active compounds. However, reported enantioselective syntheses of these molecules are indirect and often require multiple steps. Herein, we report a general asymmetric route for the one-pot synthesis of chiral γ-branched amines through the highly enantioselective isomerization of allylamines, followed by enamine exchange and subsequent chemoselective reduction. This protocol is suitable for establishing various tertiary stereocenters, including those containing dialkyl, diaryl, cyclic, trifluoromethyl, difluoromethyl, and silyl substituents, which allows for a rapid and modular synthesis of many chiral γ-branched amines. To demonstrate the synthetic utility, Terikalant and Tolterodine are synthesized using this method with high levels of enantioselectivity.

Biologically active compounds often contain a chiral centre in proximity of amine groups. Here, the authors developed a strategy involving asymmetric isomerization of allylic amines, enamine exchange and chemoselective reduction for the one-pot highly enantioselective synthesis of gamma-branched amines.